The Dieckmann cyclization of 1,3,4, 6-tetracarbethoxy-3-methylhexane gave after hydrolysis of the intermediate β-keto ester, 3-methyl-3-carboxy-cyclopentanone-2-β-propionic acid. The analogous 1,3.4, 5-tetracarbethoxy-3-methylpentane, which could theoretically cyclize to give either a cyclopentanone or a cyclohexanone derivative, gave exclusively the latter one. © 1954, American Chemical Society. All rights reserved.